1. Field of the Invention
This invention relates to photography and more particularly to artificial photoflash illumination provided for photographic exposure.
2. Description of the Prior Art
It is, of course, well known in the photographic art to employ filters of various types in combination with photoflash lamps. However, the previous efforts in this direction have had purposes quite different than the present invention, as will be apparent from the discussion hereinafter presented.
By far the most common reason for providing photoflash filtration in the prior art has been for the purpose of altering the spectral composition of the radiation emitted by the photoflash lamp so as to duplicate as closely as possible the characteristics of average daylight. The light transmitted by such a filter is thereby capable of being employed for photographic exposures of film normally intended for daylight exposure under conditions where such film would not otherwise be usable, and is especially advantageous in color photography for providing more accurate color reproduction and balance. This process of adjusting the spectral characteristics of artificial illumination to a standard for use in color photography is designated herein and in the art as "color correction". For examples of such color correcting filters, see U.S. Pat. Nos. 2,269,984 and 2,465,068.
Photoflash lamps have also been provided in the prior art with filters which are essentially opaque to the visible radiation emitted therefrom while permitting the transmission of invisible radiation such as infrared or ultraviolet radiation. These filtered photoflash lamps are especially suitable for so-called "black out" photography employing special films having sensitivity to such invisible radiation. See, for example, U.S. Pat. No. 2,337,485.
It is known that over half of the total radiation emitted from most high temperature bodies such as the sun, tungsten filaments, carbon arcs, photoflash lamps, and the like, lies in what is designated as the near-infrared region of the radiation spectrum, i.e., the region from about 700 nanometers (hereinafter abbreviated as nm) to about 2,000 nm in wavelength. As an example, some 65% of the radiation emitted from a true 4,700.degree. K black body radiator, similar to a typical photoflash light source, is in the aforementioned near-infrared region.
On the other hand, the sensitivity of conventional color negative emulsions is, for obvious reasons, confined only to the region of visible radiation, i.e., from about 400 nm to about 700 nm in wavelength. In other words, the film "sees" only about 35% of the total radiant energy emitted from a typical photoflash lamp.
Accordingly, it may be seen that a large proportion of the energy emitted by common photoflash lamps serves no useful purpose with respect to photographic illumination. Although there is no evidence as yet that this radiation contributes to undesirable side effects, or possible harm to the eye of the subject being photographed, the conservative position would be to limit the radiation of photoflash lamps to only the radiation used by the photographic process.